Cradle cap cures?

David has a bad case of this flaky dandruff. That's how it started out but now it is forming little red ridges and crusts. I don't know if I've made it worse by softening with oil and then scraping off with a fine comb? To me it looks less like cradle cap and more like psorriasis, which I suffer from. I've decided to leave it for a while and see if it improves on it's own. Baby clinic staff told me to use Betadine Shampoo or some mild cortisone application. I could not get any cortisone as the pharmacist said I need a script nor could I find the special shampoo at Clicks... but would like to try it. That brings me to the question of how often should I be washing his hair?